This invention relates to a toner for developing electrostatic images in an image forming method such as electrophotography, electrostatic printing or electrostatic recording.
In a dry copying method, an electrostatic latent image on a photosensitive medium is developed with a toner composed of a binder and a coloring agent. The developed toner image is transferred to a transfer member such as paper and fixed there.
Toner is generally produced by a grinding method or a polymerization method. In the former method, a blend of a binder, a coloring agent and other optional additives is kneaded at a temperature higher than the softening point of the binder. The kneaded product is then solidified, crushed, pulverized and sieved to have a desired particle size distribution, thereby obtaining a toner (hereinafter referred to ground toner). In the latter, polymerization method, a raw material monomer is polymerized in the presence of a coloring agent and any other optional additives to form fine toner beads. Thus, the toner of the latter type is not subjected to grinding or pulverization.
Known ground toner has a problem because part of the toner image on a photosensitive medium remains untransferred in the image transferring step. Thus, it is a general practice to remove such residual toner in a cleaning step. In view of the recent demand for light weight, compact-sized copying machines, however, it is desired that the cleaning be performed with a simple mechanism. Hence, there is a strong demand for a toner which ensures the efficient image transfer. The provision of such a toner with a high image transfer efficiency is essential in the case of an image forming apparatus in which the image transfer is effected by bringing an image transfer medium into pressure contact with an image-bearing photosensitive medium while applying a bias voltage thereto, since the toner image when pressed to the photosensitive member is more tightly held thereto as compared with a case where no pressure is applied to the toner image.
The toner obtained by the polymerization method has a serious problem because the content of the coloring agent cannot be increased to a desired level. In addition, residual toner remaining on a photosensitive medium after the image transfer is difficult to be removed in the succeeding cleaning step.